1. Field of the Invention
This invention relates to a power transmission system for a vehicle, and more particularly to a power transmission system for a vehicle having a first power transmission path in which engine output power is transmitted to the driving wheels through a reduction gear and a second power transmission path in which engine output power is transmitted to the driving wheels through an infinitely variable gear.
2. Description of the Prior Art
As disclosed in Japanese Unexamined Patent Publication Nos. 63(1988)-176862 and 2(1990)-24044, there has been known a power transmission system for a vehicle which comprises a first power transmission path having a reduction gear and a second power transmission path having an infinitely variable gear and in which one of the first and second power transmission paths is selected according t the running condition of the vehicle.
For example, when the first power transmission path is selected, the starting acceleration performance can be improved by virtue of torque increasing function of the torque convertor, and when the second power transmission path is selected, a proper transmission ratio is obtained according to the running condition without causing shift shock by virtue of the infinitely variable gear.
In either of the power transmission systems disclosed in the above identified patent publications, a V-belt type infinitely variable gear is employed as the infinitely variable gear and a reduction gear comprising spur gears is employed as the reduction gear.
The V-belt type infinitely variable gear comprises a driving pulley mounted on an input shaft, a driven wheel mounted on an output shaft and a V-belt passed around the driving and driven pulleys, and in the V-belt type infinitely variable gear, the transmission ratio is changed by changing the widths of the peripheral grooves of the pulleys by means of hydraulic actuators. When such a V-belt type infinitely variable gear is employed, the diametrical size of the power transmission system is inherently enlarged since the input shaft and the output shaft of the V-belt type infinitely variable gear are disposed in parallel to each other. Further when the reduction gear comprises spur gears, the diametrical size of the power transmission system increases as the reduction gear ratio increases.
The problem is more serious in a front-engine front-drive vehicle since a tunnel portion which is formed in a floor panel of the passenger room and through which the power transmission system extends rearward must be large in diameter.